In recent years, in a pulse radar apparatus, in order to further improve a target detection precision, an adaptive array antenna is incorporated, and so-called adaptive null steering is performed. The adaptive null steering is a process of applying weight control to a phase and an amplitude of a reception signal in the adaptive array antenna, thereby forming a reception synthetic beam such that the directivity in a direction in which an undesired wave comes in may become zero (null). In the adaptive array antenna which is used for this purpose, it is required to execute weight control so that the formation of the above-described reception synthetic signal can be properly performed even in an environment in which many delay signals are coming, or in an environment in which a clutter or an undesired wave is present.
This being the case, in the adaptive array antenna, attention has been paid to a weight control method in which Space Time Adaptive Processing (STAP) is adopted. The Space Time Adaptive Processing (STAP) is characterized in that SINR (Signal to Interference plus Noise Ratio) can be improved, and beam formation, in which the directivity in a direction in which an undesired wave comes is close to zero (null), can be performed.
In the Space Time Adaptive Processing (STAP), the following process is performed. To begin with, a target reflective signal is received by a plurality (N) of antennas (elemental antennas, i.e. channels) which are arranged in an array. The received signal is stored at a corresponding cell position of all process range cells, which are formed such that range cells with a width corresponding to a reception pulse width are formed to be continuous with a predetermined length on a time axis. Then, a covariance matrix is calculated from the data of range cells, which are obtained by excluding, from the stored data, range cells (also referred to as “process-adaptive range cell”) which presumably include a target signal, or in other words, from the data of range cells which are presumably formed of an undesired wave alone. At last, in a beam synthesis circuit, weight control is applied to the antenna reception signal by using an adaptive weight which has been calculated based on the covariance matrix.
In the weight control in this space time adaptive processing method, a weight calculation for each range cell is performed in the weight calculation circuit. Instead of a full-DOF (degrees-of-freedom) method which calculates a weight by using all reception pulses, there are known a pre-Doppler process which makes applicable use of the concept of DPCA (displaced phase center antenna) which reduces the size (the number of orders) of the matrix in order to decrease the processing time of the weight calculation, a Post-Doppler process which performs a weight calculation after selecting m banks which have been subjected to a Doppler filtering process, a beamspace process which performs a weight calculation after the execution of a beamspace process, and a beamspace Post-Doppler process in which these processes are combined. However, in these processes, there occurs a region of a speed, in which the performance relating to a target Doppler frequency deteriorates, and it is difficult to obtain a good SINR characteristic.
As has been described above, in the space time adaptive processing by the weight control of the adaptive array antenna which is used in the conventional radar apparatus, when the Post-Doppler process is applied to the weight calculation for reducing the directivity in the direction of the undesired wave to null, there occurs the region of speed in which the performance relating to the target Doppler frequency deteriorates. Thus, it has been difficult to obtain good SINR characteristics.